A light emitting diode (LED) that uses such a compound semiconductor as AlGaAs, AlGaInP and AlGaInN as its material is expected to become the energy-saving, long-life illumination/display light source to take over the existing lighting equipment such as incandescent bulbs and fluorescent lamps, and research and development for its widespread use are being strategically promoted around the world.
The energy conversion efficiency of a light emitting diode is generally decided based on the product of internal quantum efficiency and efficiency of extracting light from semiconductor to the outside, i.e., light extraction efficiency. The internal quantum efficiency has seen dramatic improvement based on the advancement of crystal growth technology in recent years. For example, a red LED made of an AlGaInP-based material having a light emission wavelength of approximately 650 nm with an internal quantum efficiency of nearly 100% has already been put into practical application.
Moreover, a device of an InGaN-based blue LED with internal quantum efficiency of 70% to 80% has also been reported. Meanwhile, it is extremely difficult to efficiently extract light (spontaneous emission) generated within the semiconductor to the outside (into the air), and it would not be an overstatement to say that this is the biggest factor which is inhibiting the improvement in the luminous efficiency of LEDs.
There are three primary factors in the foregoing problem. Specifically, 1) total internal reflection of light at the semiconductor/air interface caused by the high refractive index of the semiconductor material, 2) shielding of light by the Ohmic electrode, and 3) absorption of light by the absorptive substrate. For example, in the case of a flat substrate device, due to the total internal reflection at the interface, the light that can be extracted outside is light that enters the interface at an angle that is smaller than the critical angle of the total internal reflection, and the amount of such light is generally only several percent (2 to 4%) of the light that was generated in the active layer.
In order to improve the light extraction efficiency of LEDs, various technologies have been developed to date. For example, as technologies for suppressing the total internal reflection at the interface, there are, for example, the methods of (1) encapsulating the LED chip with resin having a higher refractive index than air, (2) using a mechanical method to process the crystals into a special shape such as an inverted pyramid (refer to Non-Patent Document 1), (3) controlling the irradiation mode of the light with microcavities or a photonic crystal structure (refer to Patent Document 1), and (4) forming a metal mirror with a high reflectance on one side of a thin film crystal serving as the main body of the light emitting diode that is separated from the substrate and intentionally forming microasperities on the other side, and subsequently utilizing the modulation effect of the angle of reflection of the microasperities and the multiple reflection at the interface between the metal mirror and air to increase the ratio of light that enters the air interface at an angle that is smaller than the critical angle of the total internal reflection. But, even using these technologies does not make it easy to obtain a light extraction efficiency that exceeds 50%.
In addition, the special geometry shaping of crystals based on the mechanical method and the production of microcavities or a photonic crystal structure entail problems such as (1) the production process being complex and expensive, and (2) not being suitable for large area (high power) devices. In particular, regarding the production cost, it is absolutely imperative for the production cost of LEDs to be lowered by one digit or more than the current production cost to realize the widespread use of LEDs as a solid-state illumination device. Accordingly, the development of light extraction technology in which the production process is simple and capable of easily achieving a light extraction efficiency of 50% or higher is being strongly demanded.
The present inventors have developed a new type of light emitting diode in which the current injection area and the light emitting area (crystal plane with low band gap energy) are separated spatially by selectively forming a metal electrode for current injection on a crystal plane with a high band gap energy by utilizing the behavior in which the band gap energy of the semiconductor epitaxial layer grown on the non-planar substrate having a plurality of crystal planes depends on the crystal plane orientation (refer to Patent Document 2).
In this device, since the carrier injected from the crystal plane with a high band gap energy emits light after moving to a crystal plane with a low band gap to energy, it was possible to considerably inhibit the shielding of light by the electrode. In the demonstration experiment using a GaAs/AlGaAs-based material, a light extraction efficiency of approximately 15% (no resin encapsulation) was obtained using a process that is simpler than the conventional technologies. However, this technology hardly showed any effect against the problem of the absorption of light by the absorptive substrate, and the effect against the total internal reflection at the interface was also insufficient.    [Patent Document 1] Japanese published unexamined Application No. 2008-311687    [Patent Document 2] Japanese published unexamined Application No. 2007-214558    [Non-Patent Document 1] M. R. Krames, M. Ochiai-Holcomb, G. E. Hofler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang, S. A. Stockman, F. A. Kish, T. S. Tan, C. P. Kocot, M. Hueschen, J. Posselt, B. Loh, G. Sasser, and D. Collins, “High-power truncated-inverted-pyramid (AlxGa1-x)0.5In0.5P/GaP light emitting diodes exhibiting >50% external quantum efficiency”, Applied Physics Letters, Vol. 75 (1999) 2365-2367.    [Non-Patent Document 2] H. Weman, E. Martinet, A. Rudra, and E. Kapon, “Selective Carrier Injection into V-groove Quantum Wires”, Applied Physics Letters, Vol. 16 (1998) 2959-2961.